Abstract

Surge and swab pressures, which could cause lost circulation or gas kicks and blowout, result from running or pulling a pipe in a borehole. Recent downhole measurements indicate that these pressures differ considerably from the pressures anticipated by traditional computational models which compute such pressures on the basis of steady state flow.

In this paper, a new dynamic surge and swab model is presented that is based on the physics of the transient nature of the phenomenon. It treats friction as a distributed parameter and considers various parameters in hole geometry, hole expansion, varying trip velocity, return area for tricone and diamond drill bits, plugged jets, and the mud properties. The computer program for the model not only predicts the maximum surge and swab pressures or their variation with time at various points in the borehole, but also computes the safe maximum trip speed for a specified pressure margin. Computer studies for running a casing, liner, and drill string in several actual wells show the model to have fast computer run times. The results of some of these studies are reported in the paper. The program is both interactive and user-friendly, and is a significant improvement over the existing models.

Introduction

In drilling, a frequent and time-consuming operation is the running of pipes in a borehole. Tripping a bit and running a casing or liner are a few common instances requiring such an operation. When pipe is pulled out of a hole at fast speeds to save time, it may generate large swab, or negative surge, pressures inside the hole, resulting in kicks. On the other hand, if pipe is run in too fast, it generates large surge pressures which can fracture the formation and result in lost circulation.

In view of its importance, the problem of predicting surge and swab pressures and the safe predicting surge and swab pressures and the safe trip speeds has been studied since the early days of drilling. Starting in the early 1930's, several models and techniques have been proposed for the prediction of such pressures. Almost all the traditional techniques consider these pressures to be due to the steady-state flow, pressures to be due to the steady-state flow, and essentially compute the pressure through the frictional pressure loss in the annulus. These techniques would give correct predictions for the surge and swab pressures, were it not for the following facts. The fluid flow in the pipe and annulus is unsteady because the pipe pipe and annulus is unsteady because the pipe is set in motion and then brought to a stop as soon as a complete joint of casing or a stand of drill pipe is run in or pulled from the hole. Further, the fluid is compressible, and so are the various conduits through which it flows under pressure. These factors have a significant effect pressure. These factors have a significant effect on the flow and thus on the surge and swab pressures.

It is therefore not surprising from recent downhole measurements that the actual surge and swab pressures differ considerably from the pressures anticipated by the traditional pressures anticipated by the traditional computational techniques. The physics of the surge and swab phenomenon dictate that a dynamic model, based on the unsteady flow, must be developed for the prediction of surge and swab pressures. This fact was recognized by Lubinski in the early 1960's, and a dynamic model, based on a graphical technique due to Bergeron was proposed for the solution of this problem. However, besides other limitations, this model considered the friction as a "lumped" parameter and used artificial orifices, placed at several points along the flow, to represent frictional pressure losses.

In this paper, a new dynamic surge and swab model is presented which not only predicts the maximum surge and swab pressures or their variation with time at the bottom of the hole, casing-shoe, or at any other point in the borehole for a given trip speed, but also computes the maximum safe trip speed for a specified pressure margin or effective circulating density (ECD).

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